Enhanced process for the purification of anhydrous hydrogen chloride gas

ABSTRACT

The present invention relates to a process for purifying anhydrous hydrogen chloride gas (“aHCl”), and preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process. In the process of the present invention, the content of chlorinated organics may be reduced from up to 1000 ppm by volume to below 10 ppb by volume levels. Generally, the process of the invention allows for chlorinated organic levels to be reduced to from 1 to 100 ppb, rendering the treated hydrogen chloride gas usable in a catalytic oxychlorination process or a Deacon process. The treated gas is also suitable for absorption in water or dilute hydrochloric acid.

BACKGROUND OF THE INVENTION

The present invention relates to a process for purifying anhydroushydrogen chloride gas (“aHCl”), and preferably the anhydrous hydrogenchloride gas recovered from an isocyanate production process. In theprocess of the present invention, the content of chlorinated organicsmay be reduced from up to 1000 ppm by volume to below 10 ppb by volumelevels. Generally, the process of the invention allows for chlorinatedorganic levels to be reduced to from 1 to 100 ppb, rendering the treatedhydrogen chloride gas usable in a catalytic oxychlorination process or aDeacon process. The treated gas is also suitable for absorption in wateror dilute hydrochloric acid.

A number of important chemical processes generate aHCl as a byproduct.Examples of such processes include chlorination processes, silaneproduction processes and phosgenation processes. Because large amountsof aHCl can not be disposed of, one of the challenges encountered witheach of these processes is purification of the aHCl generated to obtaina usable technical product or raw material for other processes. Severalprocesses for purifying aHCl generated during production processes havebeen proposed. Thermal treatment of the aHCl at temperatures of up to800 to 1600° C. is disclosed in U.S. Pat. No. 5,126,119. Fullcondensation and distillation under elevated pressure is disclosed inU.S. Pat. No. 4,935,220. The processes disclosed in these patentsrequire high amounts of energy and expensive equipment.

Treatment of aHCl at pressures of 5 to 20 bar absolute and finaltemperatures below −20° C. is disclosed in U.S. Pat. No. 6,719,957. Theprocess disclosed in the '957 patent results in contaminant levelsoccasionally unacceptable for use in vinyl chloride production. Thecontaminant level achieved is always unacceptable for use in Deaconprocesses.

In the commercial phosgenation processes for the production ofisocyanates such as TDI (toluene diisocyanate, MDI (diphenylmethanediisocyantes) and HDI (hexamethylen diiscocyanate), two moles of aHClare formed per isocyanate group produced. This large quantity ofby-product must be used in a secondary process.

One such secondary process is the production of muriatic acid. However,the volume of HCl byproduct produced often exceeds the market demand.Another alternative is to use the aHCl in a catalytic oxychlorinationprocess with ethylene to produce ethylene dichloride and finally vinylchloride as the commercial product. This catalytic process is verysensitive to traces of organic compounds, particularly (chloro-)aromatic compounds which can deactivate the catalyst employed.

Another secondary process is the Deacon process, which produces chlorineand water by passing gaseous HCl and oxygen over a transition metalcatalyst. This process is very sensitive to traces of some contaminants,such as sulfur and some organic compounds, which over time can lead tocatalyst deactivation and/or plugging of reactors, which in turn canlead to unwanted by-product formation.

The most commonly used solvents in isocyanate production arechlorobenzene and dichlorobenzene (See G. Oertel, Polyurethane Handbook,page 66 (Carl Hanser Verlag, Munich (1985)). The aHCl recovered from thephosgenation process is saturated with these chloro-aromatics. Deepchilling of the aHCl gas can reduce the chloro-aromatic content, but notto the necessary level. Another complicating factor is the high meltingpoint of dichlorobenzene (o-isomer: −17.5° C., p-isomer: +52.8° C.),which limits the usefulness of this approach. Low pressure phosgenationprocesses such as those described in G. Oertel, Polyurethane Handbook,p. 66 (Carl Hanser Verlag, Munich (1985)), which yield aHCl gas atpressure ranging from atmospheric to below 5 bar, will, even with deepchilling, contain chloro-aromatics in a concentrations of from severalhundred ppm to 1000 ppm.

The present invention has several objects: i) a process for the removalof one or more contaminants from hydrogen chloride gas, ii) a processfor separating small quantities of high boiling material, e.g., (chloro)aromatic compounds from large volumes of anhydrous HCl gas; and, iii) aprocess for reducing the concentration of contaminants such as(chloro)aromatic compounds in anhydrous HCl gas to <100 ppb.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a flow diagram for the presentinvention.

FIG. 2 schematically illustrates a second embodiment of the presentinvention.

FIG. 3 schematically illustrates a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is broadly directed to a cooling and distillationprocess to remove contaminants having boiling points higher thanhydrogen chloride from a hydrogen chloride-containing gas comprising:

-   -   a) compressing said hydrogen chloride-containing gas,    -   b) cooling the resultant compressed gas in a first heat        exchanger resulting in a first condensate stream and a first gas        stream, wherein said compressed gas is cooled to a temperature        low enough to partially condense said contaminants and at a rate        sufficiently low that fog formation is prevented,    -   c) feeding said first gas stream from said first heat exchanger        to a distillation column having a top portion and a bottom        portion to a point between said top portion and said bottom        portion, to cause mass transfer between liquid and gas and to        thereby concentrate the contaminants in the bottom portion of        said column and hydrogen chloride gas in the top portion of said        column,    -   d) feeding said hydrogen chloride gas from said top portion to a        second heat exchanger whereby the hydrogen chloride gas is        partially condensed to form a second condensate stream and a        second gas stream,    -   e) feeding said second condensate stream to said top portion of        said column to provide reflux to said column,    -   f) feeding said first condensate stream to said distillation        column below the point where said first gas stream is fed,    -   g) feeding said second gas stream from step d) to said first        heat exchanger as cooling medium,    -   h) recovering purified hydrogen chloride gas from said first        heat exchanger, and    -   i) feeding said contaminants from the bottom portion of said        column to a collection vessel.

In the compression step (step a)), the gas is preferably compressed to apressure of from 5 to 30 bars absolute.

The contaminants contained in the gas stream are preferably chlorinatedaromatic compounds. In one preferred embodiment, the gas stream alsocontains a contaminant with an intermediate boiling range between thehydrogen chloride boiling point and the chlorinated aromatic compoundboiling point. The intermediate contaminant is removed from thedistillation column, is subsequently depressurized, and is discarded. Inone especially preferred embodiment, the intermediate contaminant isphosgene.

In the cooling step (step b)), the incoming contaminated gas is cooledslowly. The temperature difference between the cooling wall of the firstheat exchanger and the inlet gas temperature is preferably between 0.5and 40° C., and most preferably in the range of from 5 to 25° C. Thetemperature of the compressed gas is preferably reduced to a temperatureof from +10 to −25° C. in the cooling step (step b)).

In one embodiment of the invention, in step f), the condensate stream(of step b)) is fed to a separation vessel (or vessels) used to trapsolids. If multiple vessels are used, one vessel can be used to collectsolids while overflowing condensate from the vessel being fed to thedistillation column at a point below the point where the first gasstream is fed, while the other vessel is depressurized to enablecollected solids to be purged to waste.

In another preferred embodiment, step e) comprises:

-   -   e1) feeding said second condensate stream to one or more        separation vessels used to trap any solids present and to form a        solids stream and a third condensate stream,    -   e2) feeding said third condensate stream to said top portion of        said column to provide reflux to said column.        The solids collected in the solids stream can be purged to        waste.

In an other preferred embodiment, step i) comprises i1) feeding liquidfrom the bottom portion of said column to a reboiler to generatestripping vapors for the bottom portion of the column, and wherein thereboiler heats the said liquid at low heat flux so as to prevent foamingaction and i2) removing any remaining liquid from the reboiler to acollection vessel for disposal. Preferably, from 5% to 95% of the liquidreaching the reboiler is evaporated. Most preferably, the reboilerdesign prevents the formation of foams and has a heat flux of from 500to 20,000 BTU/hr/ft² as a lower limit and from 3,000 to 30,000BTU/hr/ft² as a higher limit. In any even more preferred embodiment, aportion of the liquid removed from the reboiler comprises hydrogenchloride and contaminants and is sprayed into the gas stream being fedto the first heat exchanger, most preferably in amount of from 1 to 25%by weight of the weight of the incoming gas stream.

The temperature of the gas being fed into the distillation column ispreferably reduced to a temperature of from 0 to −35° C. during thedistillation step.

In another preferred embodiment, the purified hydrogen chloride gas fromthe first heat exchanger is further purified by treatment with activatedcharcoal.

In a second broad embodiment, the invention comprises

-   -   a) compressing said hydrogen chloride-containing gas,    -   b) cooling the resultant compressed gas in a first heat        exchanger resulting in a first condensate stream and a first gas        stream, wherein said compressed gas is cooled to a temperature        low enough to partially condense said contaminants and at a rate        sufficiently low that fog formation is prevented,    -   c) feeding said first gas stream from said first heat exchanger        to a distillation column having a top portion and a bottom        portion to a point between said top portion and said bottom        portion, to cause mass transfer between liquid and gas and to        thereby concentrate the contaminants in the bottom portion of        said column and hydrogen chloride gas in the top portion of said        column,    -   d) feeding said hydrogen chloride gas from said top portion to        one side of a third heat exchanger and feeding said contaminants        from the bottom portion of said column to the other side of said        third heat exchanger to flash against and cool the hydrogen        chloride gas passing through said third heat exchanger, whereby        the following streams are formed:        -   1) a second gas stream containing contaminants,        -   2) a contaminant stream,        -   3) a third cooled gas stream, and        -   4) a second condensate stream,    -   e) feeding said third gas stream to a second heat exchanger        whereby the hydrogen chloride gas is partially condensed to form        a third condensate stream and a fourth gas stream,    -   f) combining said second condensation stream and said third        condensation stream and feeding the resulting combined stream to        said top portion of said column to provide reflux to said        column,    -   g) feeding said first condensate stream to said distillation        column below the point where said first gas stream is fed,    -   h) feeding said fourth gas stream from step d) to said first        heat exchanger as cooling medium, and    -   i) recovering purified hydrogen chloride gas from said first        heat exchanger.

In addition, any of the various preferred parameters and embodimentsdescribed above can be used with this second broad embodiment. Forexample, in step g), the condensate stream (of step b)) can be fed to aseparation vessel (or vessels) used to trap solids. If multiple vesselsare used, one vessel can be used to collect solids while overflowingcondensate from the vessel being fed to the distillation column at apoint below the point where the first gas stream is fed, while the othervessel is depressurized to enable collected solids to be purged towaste. Additionally, step f) can comprise:

-   -   f1) combining said second condensation stream and said third        condensation stream,    -   f2) feeding the combined condensate stream to one or more        separation vessels used to trap any solids present and to form a        solids stream and a fourth condensate stream, and    -   f3) feeding said fourth condensate stream to said top portion of        said column to provide reflux to said column.        The solids collected in the solids stream can be purged to        waste. Similarly, the column can be provided with a reboiler,        with the liquid from the reboiler being fed to the flasher. A        portion of the liquid removed from the reboiler which contains        hydrogen chloride and contaminants can be sprayed into the gas        stream being fed to the first heat exchanger.

By following the present invention, fog (or aerosol) formation isavoided by controlling the cooling rate of the incoming gas, and by theuse of a condensate spray to promote more homogeneous cooling.

The present invention provides an enhanced method of purifying acontaminated hydrogen chloride stream by using a modified cooling anddistillation process. Small quantities of high boiling contaminants,e.g. chlorinated aromatic hydrocarbons, can be removed down to aconcentration of 10 ppb in the purified gas. In particular, this processworks well for purifying byproduct streams created by isocyanateproduction processes, which coproduce with the isocyanate, large volumesof anhydrous hydrogen chloride gas with contaminants includingmonochlorobenzene and dichlorobenzenes (ortho, meta and para isomers).

The process of the invention will now be further described withreference to the drawings. Numerals and letters in the drawings refer tothe same devices and streams.

As shown in FIG. 1, the contaminated hydrogen chloride gas (shown asstream A) enters compressor 1, exits the compressor and enters the firstheat exchanger 2. As it passes through the first heat exchanger, thecompressed gas is cooled to a temperature low enough to partiallycondense the contaminants and at a rate sufficiently low that fogformation is prevented. Two streams flow from the first heat exchanger afirst condensate stream C and a first gas stream B. The first gas streamB is fed to a distillation column 3 at a point between the top andbottom of the column. In the distillation column, mass transfer occursbetween liquid and gas, with the contaminants being concentrated in thebottom portion of the column, and hydrogen chloride gas beingconcentrated in the lower portion of the column. The hydrogen chloridegas is fed (stream D) from the top portion of the column to a secondheat exchanger 4 (that is provided with an appropriate coolant viastream shown in the figure as arrows entering one side of the exchangerand exiting on the other side) wherein the gas is partially condensed toform a second condensate stream E and a second gas stream F. The secondcondensate stream E is fed back to the top portion of the column toprovide reflux to the column. The first condensate stream C is fed tothe column at a point below the first gas stream B is fed. The secondgas stream F is fed back to the first heat exchanger as cooling medium.Purified hydrogen chloride gas is recovered via stream G from the firstheat exchanger and the liquid bottoms (that contain concentratedcontaminants) of the column are fed via stream H to a collection vessel(not shown) for subsequent disposal.

The configuration in FIG. 2 is similar to that in FIG. 1 with severaladded improvements shown. FIG. 2 illustrates the process as if all theimprovements were used. Of course, the artisan will recognize that notall the improvements must be used. As shown, the inlet gas stream Aenters compressor 1 exits the compressor and enters the first heatexchanger 2. As it passes through the first heat exchanger, thecompressed gas is cooled to a temperature low enough to partiallycondense the contaminants and at a rate sufficiently low that fogformation is prevented. Two streams flow from the first heat exchanger afirst condensate stream C and a first gas stream B. The first gas streamB is fed to a distillation column 3 at a point between the top andbottom of the column. In the distillation column, mass transfer occursbetween liquid and gas, with the contaminants being concentrated in thebottom portion of the column, and hydrogen chloride gas beingconcentrated in the lower portion of the column. The condensate from thefirst heat exchanger flows (stream C′) into a solids trapping vessel 5.If more than one vessel is used, they may be used interchangeably. Inthe separation vessel two streams result, a solids stream C″ that can becollected and purged to waste and a third condensate stream C. The thirdcondensate stream (without solids) C is fed as a liquid feed to thedistillation column at a point below the point where the first gasstream is fed.

The concentrated hydrogen chloride gas from the top portion of thecolumn is fed via stream D to a third heat exchanger 9. The bottomsstream H from the distillation column can be split into two streams, H′and H″ which contain concentrated amounts of the contaminants. The H″stream can be pumped via pump 10 back into the inlet of the HCl gasentering the first heat exchanger. The H′ stream can be flashed againstthe concentrated hydrogen chloride gas entering the third heatexchanger. This step can result in several streams—i) a gas stream Jcontaining hydrogen chloride (at a lower pressure) and, in the case of astarting gas from an isocyanate production facility, phosgene (thisstream can be collected and used again in another appropriate process),ii) a stream J′ containing mainly organic contaminants (that are thencollected and disposed), iii) a gas stream D′ that is fed to the secondheat exchanger 4 and iv) a condensate stream E″. Stream E″ can becombined with the second condensate stream E and fed to the top portionof the column 3 to provide reflux to the column. Alternatively, streamE″ can be combined with the second condensate stream E and fed to acollection vessel 6 where solids are collected and discarded via streamE′″ with the overflow condensate from the collection vessel being fedvia stream E′ back to the top portion of the column 3 to provide refluxto the column. The second gas stream F is fed back to the first heatexchanger as cooling medium.

FIG. 2 also shows a side draw-off stream I from the distillation columnto remove intermediate boiling contaminants. This stream can be fed toan activated charcoal bed 8 to remove organics, resulting in stream I′which is led off to disposal.

FIG. 2 also shows purified gas stream G being fed to an activatedcharcoal bed 7, resulting in a stream G′ of purified hydrogen chloridegas.

The embodiment shown in FIG. 3 is identical to that shown in FIG. 2,except that the bottom liquids of the distillation column are fed viastream H to a reboiler 11 (that is provided with an appropriate coolantvia stream shown in the figure as arrows entering one side of theexchanger and exiting on the other side) to generate stripping vaporsthat are fed via stream K to the bottom portion of the column. Thereboiler heats the bottoms liquid at low heat flux to prevent foamingaction. The condensate stream H′″ from the reboiler can either becollected and discarded or can be sent to either the third heatexchanger (stream H′) or back to the inlet of the first heat exchanger(stream H″).

The compressor A can be of any kind of equipment capable of increasingthe pressure to from about of 5 to 30 bar absolute and preferably above12 bar absolute. Preferred compressors include piston compressors, screwcompressors, optionally with oil injection, and centrifugal compressors.The final pressure of the gas must be adjusted so as to overcome thepressure drop in overall system.

Once compressed, the gas enters the first heat exchanger at which pointgas condensate spray mixes with the incoming gas in the amount of fiveto twenty-five weight percent of the total amount of incoming gas. Somecondensation from this first heat exchanger can flow into solidscollection vessel(s) to catch solids. The liquid condensate from thesevessel(s) overflows and is fed to the distillation column.

The heat exchangers used in the present invention can be of any type.Shell and tube heat exchangers are preferred.

HCl offgas from isocyanate units containing monochlorobenzene,dichloro-benzene, and chlorinated methanes impurities are preferablyused as the initial gas. The gas is compressed to a pressure of 8 to 20bar, preferably 12 bar. The resulting compressed gas is fed to the firstheat exchanger for cooling to between −5 and −20° C., preferably −10°C., to partially condense impurities. The condensed impurities arepreferably first passed through a solids collection vessel to remove anysolids and then led to the distillation column as liquid feed. The gasstream from the first heat exchanger is fed to the distillation columnas gaseous feed. Overhead vapors from the distillation column passthrough a second heat exchanger and are cooled to between −18 and −30°C. (preferably −25° C.) to partially condense between 0.01 and 25%,preferably between 2 to 5%, of the inlet vapor stream to provide liquidreflux for the distillation column. The purified HCl gas from the secondheat exchanger are pumped back to the inlet gas stream to be injectedlike a spray to promote condensation in the inlet gas and to prevent theformation of an aerosol or fog. The partially purified gas now has aconcentration of from 0.1 to 100, and preferably 1 to 10 ppm organicimpurities.

The partially purified HCl gas can then be fed to an activated charcoaladsorption column for final purification to reach a final organicimpurities level of from 10 to 1000 ppb, preferably from 50 to 100 ppb.

The bottom stream of the distillation column contains most of theimpurities and is allowed to flash to a lower pressure of between 1 barand 10 bar, preferably 1.05 bar where much of the remaining HCl flashesoff and is led off to an absorption step or to waste. The remainingresidue, containing most of the impurities, is led off to incinerationor other waste treatment. The cooling effect of the bottoms stream flashcan help to cool the vapors leaving from the top of the column.

If desired, a reboiler (preferable operation between −10 and +8° C.) isprovided to return most of the condensed HCl as vapor back to the columnas stripping gas. The design of the reboiler preferably uses a heat fluxof between 1000 and 12000 BTU/hr/ft² and preferably between 2500 and4000 BTU/hr/ft².

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein withoutdeparting from the spirit and scope of the invention except as it may belimited by the claims.

1. A cooling and distillation process to remove contaminants havingboiling points higher than hydrogen chloride from a hydrogenchloride-containing gas comprising: a) compressing said hydrogenchloride-containing gas, b) cooling the resultant compressed gas in afirst heat exchanger resulting in a first condensate stream and a firstgas stream, wherein said compressed gas is cooled to a temperature lowenough to partially condense said contaminants and at a ratesufficiently low that fog formation is prevented, c) feeding said firstgas stream from said first heat exchanger to a distillation columnhaving a top portion and a bottom portion to a point between said topportion and said bottom portion, to cause mass transfer between liquidand gas and to thereby concentrate the contaminants in the bottomportion of said column and hydrogen chloride gas in the top portion ofsaid column, d) feeding said hydrogen chloride gas from said top portionto a second heat exchanger whereby the hydrogen chloride gas ispartially condensed to form a second condensate stream and a second gasstream, e) feeding said second condensate stream to said top portion ofsaid column to provide reflux to said column, f) feeding said firstcondensate stream to said distillation column below the point where saidfirst gas stream is fed, g) feeding said second gas stream from step d)to said first heat exchanger as cooling medium, h) recovering purifiedhydrogen chloride gas from said first heat exchanger, and i) feedingsaid contaminants from the bottom potion of said column to a collectionvessel.
 2. The process of claim 1, wherein the contaminants contained inthe gas stream are chlorinated aromatic compounds.
 3. The process ofclaim 2, wherein the gas stream also contains a contaminant with anintermediate boiling range between the hydrogen chloride boiling pointand the chlorinated aromatic compound boiling point.
 4. The process ofclaim 3 wherein said intermediate is phosgene and said intermediate isremoved from said distillation column.
 5. The process of claim 1 whereinthe temperature of the compressed gas is reduced to a temperature offrom +10 to −25° C. in said first heat exchanger.
 6. The process ofclaim 1 wherein in step a), the gas is compressed to a pressure of from5 to 30 bars absolute.
 7. The process of claim 1, wherein step f)comprises f1) feeding said first condensate to a separation vessel (orvessels) to trap solids and wherein a solids stream and an overflowcondensate stream are formed, f2) feeding said overflow condensatestream to said distillation column at a point below the point where thefirst gas stream is fed.
 8. The process of claim 1, wherein step e)comprises: e1) feeding said second condensate stream to one or moreseparation vessels used to trap any solids present and to form a solidsstream and a third condensate stream, e2) feeding said third condensatestream to said top portion of said column to provide reflux to saidcolumn.
 9. The process of claim 1 wherein step i) comprises i1) feedingliquid from the bottom portion of said column to a reboiler to generatestripping vapors for the bottom portion of the column, and wherein thereboiler heats the said liquid at low heat flux so as to prevent foamingaction and i2) removing any remaining liquid from the reboiler to acollection vessel for disposal.
 10. The process of claim 9, wherein thereboiler is designed to prevent the formation of foam and has a heatflux of from 500 to 20,000 BTU/hr/ft² as a lower limit and from 3,000 to30,000 BTU/hr/ft² as a higher limit.
 11. The process of claim 9, whereina portion of the liquid removed from the reboiler comprises hydrogenchloride and contaminants and is sprayed into the gas stream being fedto the first heat exchanger.
 12. The process of claim 9, wherein from 5to 95% by weight the liquid fed to said reboiler is evaporated.
 13. Theprocess of claim 1, wherein the purified hydrogen chloride gas from thefirst heat exchanger is further purified by treatment with activatedcharcoal.
 14. A cooling and distillation process to remove contaminantshaving boiling points higher than hydrogen chloride from a hydrogenchloride-containing gas comprising: a) compressing said hydrogenchloride-containing gas, b) cooling the resultant compressed gas in afirst heat exchanger resulting in a first condensate stream and a firstgas stream, wherein said compressed gas is cooled to a temperature lowenough to partially condense said contaminants and at a ratesufficiently low that fog formation is prevented, c) feeding said firstgas stream from said first heat exchanger to a distillation columnhaving a top portion and a bottom portion to a point between said topportion and said bottom portion, to cause mass transfer between liquidand gas and to thereby concentrate the contaminants in the bottomportion of said column and hydrogen chloride gas in the top portion ofsaid column, d) feeding said hydrogen chloride gas from said top portionto one side of a third heat exchanger flasher and feeding saidcontaminants from the bottom portion of said column to the other side ofsaid third heat exchanger to flash against and cool the hydrogenchloride gas passing through said third heat exchanger, whereby thefollowing streams are formed: 1) a second gas stream containingcontaminants, 2) a contaminant stream, 3) a third cooled gas stream, and4) a second condensate stream, e) feeding said third gas stream to asecond heat exchanger whereby the hydrogen chloride gas is partiallycondensed to form a third condensate stream and a fourth gas stream, f)combining said second condensation stream and said third condensationstream and feeding the resulting combined stream to said top portion ofsaid column to provide reflux to said column, g) feeding said firstcondensate stream to said distillation column below the point where saidfirst gas stream is fed, h) feeding said fourth gas stream from step d)to said first heat exchanger as cooling medium, and i) recoveringpurified hydrogen chloride gas from said first heat exchanger.